In current display systems employing pulse-width-modulation techniques, such as OLEDs, LCDs, plasmas, micromirror-based display systems and the like, the pixels of the display systems often exhibit asymmetrical switching delays in response to their driving forces. Such switching delays may arise from electromechanical responses of the pixels to the driving forces and optical responses of the components of the system to the driving forces. As a consequence, image quality is deteriorated.
As an example, in a micromirror-based display system, each pixel is a micromirror having a deflectable reflective mirror plate. The mirror plate rotates in response to an electrostatic force to different angles in opposite rotational directions. The ON and OFF operating states of the micromirror are defined based on the rotation angles. In the ON state, the mirror plate rotates to an ON state angle so as to generate a “bright” image pixel on a display target, whereas the mirror plate rotates to an OFF state angle so as to generate a “dark” image pixel on the display target.
Grayscale images can be created by turning the micromirror on and off at a rate faster than the human eye can perceive, such that the pixel appears to have an intermediate intensity proportional to the fraction of the time when the micromirror is on. This method is generally referred to as pulse-width-modulation (PWM). Full-color images may be created by using the PWM method on separate SLMs for each primary color, or by a single SLM using a field-sequential color method.
For addressing and turning the micromirror on or off, each micromirror may be associated with a memory cell circuit that stores a bit of data that determines the ON or OFF state of the micromirror. Specifically, the stored bit determines the magnitude of the electrostatic field between the mirror plate of the micromirror and the associated electrode. In order to achieve various levels of perceived light intensity by human eyes using PWM, the intensity level of each pixel of a grayscale image is represented by a plurality of data bits. Each data bit is assigned a significance. Each time the micromirror is addressed, the value of the written data bit determines whether the addressed micromirror turns on or off. Each bit's significance determines the duration of the micromirror's subsequent on or off period according to the addressing pattern. The bits of the same significance from all pixels of the image are called a bitplane. If the elapsed time the micromirrors are left in the state corresponding to each bitplane is proportional to the relative bitplane significance, the micromirrors produce the desired grayscale image.
This type of operation mechanism certainly favors prompt response of the micromirror to the electrostatic fields applied thereto. Ideally, the responses to the ON state and OFF state are symmetrical. In other words, the transition time of the micromirror from the ON state to the OFF state should be the same as the transition time from the OFF state to the ON state. Otherwise, the micromirror may not be able to accurately reproduce the desired grayscale. However, many real systems do exhibit asymmetry in the ON-to-OFF and OFF-to-ON switching times. When the micromirror has different transition time intervals for the ON and OFF state, the actual duration of the micromirror's optical on or off period is not the same as determined by the bits of the PWM. The actual grayscale produced by the micromirror deviates from the desired value.
Therefore, what is desired is a method of operating the pixels of a display system such that these asymmetrical switching delays may be compensated and an accurate grayscale level reproduced for the viewer.